supernova_poster_back

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According to the 
Annals of the Sung Dynasty (the 
Sung-shih), on the first day of the chi-ho 
reign period, during the 5th month, on the 
chi-chou, a “guest star” appeared to the south 
east of Tian-kuan. 
The guest star was so 
bright that it could be 
seen during the day-
time, and it remained 
so for 23 days. Af-
ter that, it gradually 
dimmed, finally fading 
from visibility after two years. 
Japanese records also mention the 
star.
At around the same time, half a 
world away from China in what 
we now call Chaco Canyon, in 
Northern New Mexico, an ancestor of today’s 
Hopi and Navajo painted what appears to be a 
“guest star” into a protected rock overhang. The 
star is shown next to the crescent moon, and 
a hand print, perhaps that of the artist, is also 
painted on the rock. Though this record is not 
as detailed as that of the Sung astronomers, the 
orientation of the moon and the guest star is 
what it would have been on that day when the 
Sung reported their guest star, strongly suggest-
ing that the two are the same. 
Such an impressive object, recorded in dispa-
rate cultures around the globe, must have been 
visible in Europe as well as Asia and North 
America. However, the date given in the Chi-
nese annuls, by our modern reckoning, would 
have been July 4, 1054. At that time Europe-
ans were in the throes of the Dark Ages, and 
the Norman Invasion was just a few years 
away. Perhaps they were too occupied 
with worldly concerns to mark down 
the appearance of a celestial visitor, 
or perhaps whatever record existed 
has been lost. 
In any case, no European record of the event has 
ever been found.
Since the appearance nearly a millennium ago of 
the Sung “guest star” there have been only two 
other similar objects seen in our Galaxy. One oc-
curred in 1572 in the constellation Cassiopeia. 
This was observed by the Danish astronomer Ty-
cho Brahe and bears his name. It became bright 
enough to be visible in full daylight. The other 
star appeared in the constellation Ophiuchus in 
1604 and was studied by Tycho’s student and col-
laborator, Johannes Kepler, though it was seen 
earlier by several other people. Kepler’s star, while 
not as bright as Tycho’s, was still as bright as Jupi-
ter. Since the appearance of Kepler’s star, no oth-
ers have been seen in the Galaxy. 
This does not mean, however, that no additional 
similar objects have been observed. In 1885 a new 
star appeared in the center of the Milky Way’s 
companion galaxy M31, in the constellation of 
Andromeda. It reached a peak brightness of 6-7th 
magnitude, making it easily visible in small tele-
scopes against the background glow of the galaxy 
itself. The object is important for historical rea-
sons because it was used to argue, incorrectly, that 
the great spiral nebula of Andromeda was a new 
star system, like our solar system, in the process of 
forming within our own galaxy. The astronomer 
Harlow Shapley, in a famous 1922 debate with 
Heber Curtis on the nature of the spiral nebulae, 
claimed that the appearance of the guest star in 
Andromeda was due to a “new sun” just begin-
ning to turn on. His argument was later shown to 
be wrong. Edwin Hubble measured the distance 
to the Andromeda nebula and proved it was ex-
tremely far away and, in fact, an independent sys-
tem of stars – a galaxy on a par with our own 
Milky Way. 
Though these guest stars are rare 
events in any given galaxy, the 
universe 
IntroductIon to Supernovae
An 
un-
known artist 
may have recreated the 
appearance of the Crab 
Nebula supernova in 
1054 on the side of 
a cave wall in Chaco 
Canyon, New Mexico.
contains many, many galaxies. With the ad-
vent of large telescopes in the 1920s and 30s it 
was soon noticed that guest stars could be seen 
quite often if one looked at many galaxies. The 
fact that the guest stars were nearly as bright as 
the galaxies in which they occurred meant that 
they were enormously energetic. Their great 
brightness and release of energy prompted the 
astronomer Fritz Zwicky to dub them superno-
vae, because they appeared similar to, but far 
brighter than, the “novae” seen in our galaxy. 
Supernova is the name by which we still call 
them today, though we now know they have 
nothing in common with novae except a name: 
supernovae are exploding stars, whereas novae 
are the much smaller explosion of the atmo-
sphere of a white dwarf star that is acquiring 
matter from a nearby binary companion star. 
(For more about supernova life cycle, see p.8.)
In an ironic twist, recent observations of super-
novae similar to the one seen in Andromeda 
in 1885 have allowed us to measure the vast 
size and expansion rate of the universe. To our 
great surprise, these extremely distant superno-
vae indicate that the expansion is accelerating, 
rather than slowing down. These observations 
indicate that approximately 70% of the energy 
in the universe is something never before 
observed, with properties heretofore only 
imagined in the most speculative of our 
theories of nature. Far from showing 
that the universe is small, as Shapley ar-
gued, supernovae have shown us that 
the universe is not only vast, but 
much stranger than we had 
imagined. 
If you point a telescope toward 
the patch of sky described in the 
Chinese records from 1054, just 
a few degrees north and east of 
Aldebaran, the “eye” of Taurus, the 
bull, you will find a faintly glowing 
cloud. This is the Crab Nebula. It is the 
remains of a star that exploded some 7000 
years ago. The explosion was seen on 
Earth only 1000 years ago because it was 
so distant that its light required 6000 years 
to reach us; the Sung and Chaco Canyon 
inhabitants were seeing the explosion 6000 
years after it happened. The Crab Nebula 
is a supernova remnant, the debris from 
an exploded star. It is still expanding 
today at more than 1000 km/s, (for 
more about supernova expansion 
see p.11) having slowed from an 
initial expansion speed of more than 
10,000 km/s. Inside the nebula is 
the Crab pulsar, the compact 
remnant of the core of the 
exploded star. The pulsar is a 
highly magnetized, rapidly 
spinning neutron star, a class 
of object that is among the 
most bizarre found in nature. 
A mere teaspoon of the crab 
pulsar would weigh 
more than a billion 
tons (for more about 
neutron stars see 
p.22.)
In the remainder 
of this education 
unit, you will ex-
plore the amazing properties 
of supernovae and neutron stars. 
You will also begin to learn 
about some of the 
tools scientists use 
to understand 
them.
The Crab Nebula is located just above the star mark-
ing the tip of the lower horn of Taurus, the Bull.
But that’s an illusion. Like 
all things, stars are born, live out 
their lives, and eventually die, doomed 
to fade away. Stars like the Sun, which have 
a relatively low mass, age gracefully and die 
quietly after billions of years. But massive stars, 
with more than ten or so times the mass 
of the Sun, “do not go gently into 
that good night, but instead 
rage, rage against the dying of 
the light”. They explode in 
a catastrophic detonation, 
sending their outer layers 
screaming outwards at a 
few percent of the speed 
of light: what astronomers 
call a supernova.
The seeds of a star’s ulti-
mate destruction are plant-
ed deep in its core, where its 
energy is generated. Stars are 
giant balls of gas, and when a gas 
is compressed it heats up. Be-
cause stars are so big they have 
a lot of gravity, so at the core 
of a star the pressure is intense. 
This means they get very hot, hot enough to smash 
together atomic nuclei. And when nuclei collide, 
they can stick together in a process called fusion. 
This process releases a lot of energy (in fact, it’s 
what makes hydrogen bombs explode), which 
heats up the core. In a stable star like the Sun, 
the inward crush of gravity is balanced by outward 
pressure caused by the heat.
Already we see that the mass of the star is impor-
tant: it provides the gravity needed to compressthe core. The higher the mass of the star, the more 
the core is compressed, and the hotter it can get. 
Fusion reactions depend strongly on temperature; 
the higher the temperature, the faster the reaction 
proceeds. As we’ll see, this is critical later in the 
star’s life.
Initially, the star fuses hydrogen into helium. Like 
ash in a fire, the helium builds up in the core, but it 
does not fuse because helium takes a lot more pres-
sure and heat than hydrogen does to fuse. If the star 
is massive enough, though, it can ignite helium fu-
sion in its core. The helium fuses into carbon, which 
then starts to pile up in the core. In very mas-
sive stars this process repeats again and 
again, fusing lighter elements into 
heavier ones: hydrogen to he-
lium, helium to carbon, carbon 
to neon, neon to oxygen, oxy-
gen to silicon, silicon to iron. 
The star’s core starts to look 
like an onion, with layers 
nested inside one another.
At every step, the process 
generates more heat, and the 
fusion goes ever faster. A star 
may fuse hydrogen into helium 
for millions or billions of years, 
but by the time it starts to fuse sili-
con into iron, it may take mere 
days. As iron piles up in the 
core, the star is headed for di-
saster.
Why? Because up until iron, all the fusion reactions 
have produced energy in the form of heat. That heat 
holds the star up. However, iron is different. It takes 
energy to fuse iron into heavier elements, and this 
energy must come from the star itself. When enough 
iron builds up in the core, the pressure becomes great 
enough that it starts to fuse. This robs energy from 
the star, cooling it. Worse, the fusion of iron eats 
up copious amounts of electrons, and the motion of 
these electrons was helping to hold up the star too. 
When iron starts to fuse, things go bad fast. The iron 
core collapses, since the heat and electrons holding 
it up get used to fuse the iron. In a thousandth of a 
second the tremendous gravity of the core collapses 
it down from thousands of kilometers across to a 
ball of compressed matter just a few kilometers in 
The stars in the sky seem eternal and unchanging. 
Near the end of a massive star’s life, the fusion occurs in shells 
around the core, like the layers of an onion.
Fe
H
He
C
O
Si
Why StarS explode
And what of the core? Like the life of the star 
itself, the fate of the core depends on its mass. 
In relatively low-mass stars like the Sun, the star 
never explodes at all. The core is not massive 
enough to fuse helium, so helium simply builds 
up. Or perhaps helium 
does fuse, but then 
the star is not massive 
enough to fuse the re-
sulting carbon. In any 
event, the outer layers 
of the star are blown 
off by a solar wind over 
millions of years, and 
the naked core, un-
able to generate its own 
heat, simply cools and 
fades away. A star that con-
sists of this revealed core is 
called a white dwarf.
If the core is more massive, between 1 and 3 
times the Sun’s mass then things are different. 
The pressure from the collapse slams electrons 
into protons, creating neutrons. The core shrinks 
to a size of a few kilometers across, and is com-
prised almost totally of these neutrons. The col-
lapse is halted by the neutrons themselves, which 
resist the pressure. Not surprisingly, this object is 
called a neutron star.
And for more massive cores? Even the neutrons 
cannot resist the pressure created by more than 
about 3 times the Sun’s mass when it collapses. 
The core implodes, and nothing can stop it. Its 
gravity increases hugely, and anything that gets 
too close will be drawn in, even light. It has be-
come a black hole. 
This is more than just the-
ory. By studying superno-
vae, supernova remnants, 
and other exotic objects, 
astronomers have discov-
ered all this and much 
more. If you want to con-
tinue reading about this 
and get more information, 
check out the Resources 
list in the last section of 
this poster.
These pictures shows the location of Supernova 1987a before it 
exploded (left), and during the explosion (right)
David De Martin (http://www.skyfactory.org), Digitized Sky Survey.
diameter. This is a bit like kicking the legs out 
from under a table. Just like when Wile E. Coy-
ote suddenly realizes he is no longer over solid 
ground and starts to fall, the outer layers of the 
star come rushing down. They slam into the 
compressed core at a 
significant fraction of 
the speed of light. 
This does two things: 
it sets up a huge re-
bound, sending the 
outer layers of the star 
back out, and also re-
leases a vast number 
of neutrinos, sub-
atomic particles that 
carry away most of the 
energy of the collapse. The 
gas from the outer layers 
absorbs only a small frac-
tion of these neutrinos, but that’s still a lot of 
energy: it’s like lighting a match in a fireworks 
factory. The outer layers of the star explode up-
wards, and several solar masses of doomed star 
(containing the elements that were produced 
before the explosion) tear outwards at speeds of 
many thousands of kilometers per second. 
As the star explodes, the expanding gas is so hot 
that it can undergo temporary fusion, creating 
elements as heavy as uranium. This, plus other 
radioactive elements created in the explosion, 
dumps even more energy into the gas, causing it 
to glow. The expanding gas is called a supernova 
remnant; it will expand for hundreds of thou-
sands of years, eventually cooling and becoming 
so thin it merges with the tenuous gas between 
the stars. Sometimes the gas 
from the remnant will hit and 
mix with gas that is forming 
new stars, seeding it with the 
heavy elements formed in 
the explosion. The iron in 
your blood and the calcium 
in your bones were formed 
in the supernova explosion 
of a massive star millions of 
years before the formation of 
the Earth itself.
The first and second images show emission mostly from gas at the edge of 
the remnant. As the ejected material from the supernova encounters gas that 
already existed around the star, it creates shock waves, similar to the way 
a supersonic jet makes shock waves in the air. Inside these shocks the gas 
ejected from the exploded star is slowed, compressed, and heated to millions 
of degrees. This happens because, as the expanding supernova material col-
lides with the existing gas surrounding the star, its kinetic energy – the energy 
of its motion outward from the supernova – is converted to random mo-
tions, called turbulence. The high temperature of the gas means the atoms are 
moving very rapidly, which leads to very energetic collisions between them. 
The collisions are so energetic that they kick electrons completely off of the 
atoms, that is, the atoms become ionized. When gas is this hot, the atoms 
bounce off each other at high speeds, generating X-rays. Image 2 in the large 
panels on the front of the poster depicts this sort of emission. Optical view
X-ray view
As the remnant emits X-rays it loses energy. After all, X-rays are a form of radiation, like optical light, so they carry 
away a lot of the energy of the supernova into space. That loss of energy causes the remnant to cool. After several 
tens of thousands of years the outer shocked part of the remnant has cooled to only a few thousand degrees. Gas 
at this temperature is not hot enough to produce strong X-ray emission, but it is hot enough to excite (give energy 
to) the atoms within it. When an atom absorbs energy, its electrons jump from one energy level to another, like 
someone going up a staircase. After some time, the electron then falls back 
down to a lower level, and emits energy at a very specific wavelength. This 
type of emission is called line emission. In a low density but hot gas like that 
of an old supernova remnant, the emission lines typically seen are from excited 
and ionized forms of hydrogen, oxygen, nitrogen, sulfur, and other kinds of 
atoms. This can make supernova remnants appear to our eyes to glow with 
characteristiccolors such as red or green.
This lower-temperature gas tends to be from the outer parts of the supernova 
remnant. The inner part of the remnant is still filled with million degree x-ray 
emitting gas. This is because the density inside the remnant is much lower 
than in the outer shocked parts, and the lower density causes the inner part 
of the remnant to cool much more slowly. In fact, it takes more than a mil-
lion years for the hot bubble inside a supernova remnant to cool completely. 
Because these two types of emission depend on the temperature of the gas 
emitting them, they are sometimes collectively called thermal emission.
deScrIptIon of the IlluStratIonS
The following images provide an artistic rendering of emission from a superno-
va remnant in three different energy regimes: optical, x-ray and gamma radiation. While 
the overall shape of the remnant is the same in each energy band, the processes underlying the 
emission can be quite different.
The last frame in the poster depicts a different type of radiation process 
entirely. This image shows the emission of gamma rays, the most ener-
getic type of electromagnetic radiation. The gamma rays are not emitted by 
the gas itself because even a million degrees is not hot enough to produce 
gamma rays. Instead, the emission is produced by the interaction of the 
electrons in the remnant with the magnetic field of the compact neutron 
star in its center. 
This neutron star is the remains of the core of the star that collapsed. The 
neutron star is fantastically small and dense: while it’s only a few kilometers 
across, a single teaspoon of neutron star material weighs as much as 100 
million adult elephants, or a mountain a kilometer high! It spins rapidly, 
up to thousands of times per second, and can possess extremely strong 
magnetic fields. As the star’s spin sweeps its magnetic field through the 
remnant, it picks up charged particles such as electrons like a fisherman’s 
net sweeps up fish. These particles are accelerated to speeds close to the 
speed of light as they ride along with the magnetic field.
Gamma-ray view
 When charged particles are accelerated like this, 
they emit radiation. However, the radiation is very 
different from the emission described for the first 
two panels. This type of emission, called synchrotron 
emission, does not come out at one specific energy. 
Rather it is seen in a very broad, nearly flat continu-
ous spectrum, spanning all the way from radio waves 
to gamma rays. Furthermore, in contrast to emis-
sion lines and thermal X-rays, synchrotron emission 
will not diminish as the remnant cools. It depends 
only on the spinning, magnetized neutron star, not 
on the temperature of the gas. Because of this pe-
culiar property, synchrotron radiation is referred to 
as non-thermal emission. Spinning magnetized neu-
tron stars, also called pulsars because they emit pulses 
of electromagnetic radiation, are seen to slowly lose 
their spin over long periods of time. There is a lot of 
energy in the spin of such a massive object, and if it’s 
slowing, that energy has to go somewhere. Careful 
study of the energy emitted by supernova remnants 
shows that the energy emitted by them matches the 
energy lost by the slow-
ing spin of the neu-
tron star. Therefore, 
astronomers conclude 
that late in the life of 
a supernova remnant, 
its energy comes from 
the neutron star itself. 
Examples of this kind 
of remnant are the Crab 
Nebula and the Vela su-
pernova remnant. 
However, that’s not al-
ways the case. If the 
original supernova ex-
plosion is off-center 
(not a perfectly expand-
ing sphere), it can give a “kick” to the neutron star, 
basically acting like a rocket. The neutron star can 
be ejected from the explosion at very high speeds, 
hundreds of kilometers per second. After hundreds 
of years, the neutron star can actually leave the su-
pernova remnant. In those cases, the main source of 
energy for the gas is gone. These supernova remnants 
still glow from their own heat, but they tend to be 
(but are not always) much fainter than pulsar-ener-
gized remnants.
By examining supernova remnants at these different 
energies, astronomers learn different things about 
them. Their ages, energy, chemical content, and 
much more can be determined by multi-wavelength 
observations. That’s why NASA, ESA, and other 
space agencies continue to launch high-energy ob-
servatories; it’s only when seen at these different en-
ergies that the Universe reveals its secrets.
Crab nebula
Vela Nebula
tImelIne deScrIptIon
When the supernova blast wave breaks through the surface of the 
doomed star, it sets into motion a series of events and processes which will 
literally shape the fate of the expanding gas over the ensuing millennia.
The outer layers of the star explode outward due to the blast wave created when the 
core of the star collapses. The heat and pressure from this blast are so high that nuclear 
fusion can actually take place in the material. Elements up to uranium on the periodic 
table are created, many of which are radioactive. Isotopes of cobalt, aluminum, tita-
nium, and nickel are all produced in the fireball. Expanding gas normally cools, but 
as these radioactive elements decay they produce gamma rays. The gas absorbs this 
radiation, heating it. After a week or two, much of the light emitted by the supernova 
remnant is due to this heating from radioactive decay.
As time goes on, the remnant continues to expand. During the first stage of its life, 
the gas in the remnant is dense enough that it is opaque; we only see the outer part 
of the expanding cloud. But as it expands its volume increases, so its average density 
drops. Eventually, like a fog clearing, the gas becomes transparent to visible light. We see 
deeper into the remnant, and after a few years the spinning pulsar—the collapsed core 
of the star— becomes visible. The gas inside the remnant is energized by the magnetic 
fields of the pulsar (see above). That gas is diffuse and to our eyes glows with a bluish 
hue. The gas in the outer parts of the remnant, however, has been compressed into thin 
filaments and ribbons by the shock wave, and is dominated by line emission. This gas 
glows mostly red and green. 
Over many thousands of years, the pulsar-driven gas from the inner part of the nebula 
has caught up with and merged with the outer gas. The pulsar spin has slowed, its energy 
given to the expanding remnant. All that’s visible now are the shocked filaments, thin 
wisps of gas. The remnant resembles a giant spider web.
Eventually, over tens or even hundreds of thousands of years, the gas in the remnant 
mingles with the gas between the stars in the Galaxy. The remnant is no longer a discrete 
entity, but instead has merged with the interstellar medium. Stars form from such re-
gions of gas and dust, and are thus enriched by the supernova explosion: heavy elements 
such as calcium, silicon, uranium, iron, and many others seed the nascent stars. These 
heavy elements may be necessary to form planets, and in fact the iron in our blood and 
the calcium in our bones were formed in the heat and fury of some ancient supernova. 
We owe our very existence to some long-dead star, whose remains deposited its contents 
into the cloud of gas and dust that eventually became the Sun, the Earth… and us.
What is the Gamma-ray Large Area Space Tele-
scope (GLAST)?
The Gamma-ray Large Area Space Telescope 
(GLAST) is a NASA satellite planned for launch in 
2007. GLAST is part of NASA’s Science Mission 
Directorate. Astronomical satellites like GLAST are 
designed to explore the structure of the Universe, ex-
amine its cycles of matter and energy, and peer into 
the ultimate limits of gravity: black holes. GLAST 
is being built in collaboration between NASA, the 
U.S. Department of Energy, France, Germany, Italy, 
Japan, and Sweden. The project is managed from 
NASA’s Goddard Space Flight Center in Greenbelt, 
Maryland. GLAST detectsgamma 
rays, the highest energy 
light in the electromag-
netic spectrum. 
What is XMM-Newton? 
XMM-Newton is an X-ray 
satellite launched into Earth 
orbit on December 10, 1999 
by the European Space Agen-
cy (ESA). XMM-Newton is a 
fully-functioning observatory, 
carrying three very advanced 
X-ray telescopes. They each 
contain 58 high-preci-
sion concentric mir-
rors, nested to offer 
the largest collecting 
area possible to catch 
X-rays. Unlike many other 
telescopes, which only make images of the 
objects they observe, XMM-Newton takes 
both images and spectra. This means it can 
measure the energy of the X-rays emitted by 
an astronomical object, which allows scientists 
to determine many of its physical characteristics in-
cluding temperature, composition, and density. 
Credit
This poster has been developed as part of the NASA Education and Public Outreach (E/PO) 
Program at Sonoma State University, under the direction of Professor Lynn Cominsky.
Contributors to this education unit include Professor Lynn Cominsky, Dr. Kevin McLin, Dr. 
Philip Plait, Sarah Silva, and Aurore Simonnet. We would also like to acknowledge input from 
lots of other nice people too. 
http://glast.sonoma.edu or http://xmm.sonoma.edu
To learn more about GLAST, 
and XMM-Newton education 
and public outreach, visit:
GLAST
Activity 3 – At the Heart of a Supernova Explosion
Brief overview: 
Students investigate the magnetic fields and other 
properties of pulsars and compare them to the Earth’s 
magnetic field.
Grades: 8 – 12
Activity 1 - Biography of a SupernovaBrief overview: 
Students will write an essay describing the evolution of a supernova remnant based on pictures from the Supernova Poster.Grades: 7 – 12
Activity 2 – Cra
wl of the Crab
Brief overview: 
Students use two
 images of a supe
rnova remnant 
separated in time
 by several decade
s to determine 
the expansion rat
e of the glowing g
as. 
Grades: 8 – 12
This poster ac
ompanies an 
educator guid
e which conta
ins 
the following
 activities:
Get the guide at
http://xmm.sono
ma.edu/edu/sup
ernova